1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses employing a rear focusing method having a focusing portion arranged to the rear of a varifocal portion.
2. Description of the Prior Art
Focusing of conventionalzoom lenses is, in many cases, performed by moving the frontmost lens component axially forward. This focusing method has the drawback of increasing the diameter of the frontmost or focusing component. When applied to auto-focus instruments, the combined weight of the focusing component and an operating mechanism therefor is increased, the focusing speed is lowered, and a battery of increased capacity becomes necessary.
To eliminate the above-described drawbacks, another focusing method has been proposed. This involves imparting an independent movement either into one of the lens components that is movable for variation of the focal length, or into a lens component positioned nearer the image plane than the varifocal lens portion to remain stationary during the variation of the focal length, or the so-called rear focusing method. With this, a speedup of automatic adjustment of the focusing control and a minimization of bulk and size become possible. However, such rear focusing has an alternate drawback that for one and the same object distance the position of the focusing component must be changed as the focal length varies.
FIG. 1 schematically illustrates an example of a conventional optical system employing the rear focusing method, wherein W and T represent the wide angle and telephoto ends respectively, B1, B2, B3 and B4 designate the first to fourth lens components arranged from front (object) to rear (image) respectively, and F indicates a film plane. With an object at infinity, as the focal length varies, the position of the focusing component B4 remains unchanged, as indicated by a line C1. A curve C2 represents a locus of the component B4 under the condition that for an object at a minimum distance its image is always in sharp focus at the film plane F, in other words, under the in-focus condition. As is understandable from FIG. 1, for any finite object distance, the position of the focusing component B4 varies as a non-linear function of the focal length of the entire system.
A specific example of such conventional optical system has the following numerical data listed in Table 1 for the focal lengths of the lens components, f, the shortest and longest focal lengths of the entire system, fW and fT respectively, the principal point intervals, e, with the subscript numerals numbered from front to rear, and with the subscript symbols W and T representing the wide angle and telephoto positions respectively.
TABLE 1 ______________________________________ f1 = 100 e1W = 5 e1T = 33 f2 = -40 e2W = 40 e2T = 9.73 f3 = 90 e3W = 10 e3T = 12.27 f4 = 150 fW = 78.60 fT = 160.90 ______________________________________
Japanese Laid-Open Patent Application No. SHO 56-162727, provides a means for limiting the possible position of the focusing component B4 to the hatched area enclosed by the line C1 and the curve C2. The use of such means is, however, not always of assistance in speeding up the focusing operation. This is so because zooming from the telephoto to the wide angle position causes, the in-focus position for the focusing component to shift 75% for a varifocal ratio 2:1, or 90% for a varifocal ratio 3:1, from the distance it moves from the position for an infinitely distant object. Also, the curve C2 becomes rapidly steeper as the focal lengths nears the longest one. This leads to the drawback that the movement of the lens component B4 cannot be smoothly controlled.